This invention relates to an electro-optical cell for liquid crystal display of (electric) field effect type (FE type).
There are two principal types of display apparatus for converting electrical signals into visible information by utilizing the optical anisotropy of a liquid crystal. One of these is the DS type (for example, as disclosed in British Pat. No. 1,167,486) in which a dynamic scattering phenomenon wherein a nematic liquid crystal material having negative dielectric anisotropy scatters light upon the application of an electric field is utilized for displaying. The other type is the FE type (for example, as disclosed in Japanese Laid Open Application No. 11737/1972) in which a nematic liquid crystal material having a positive dielectric anisotropy is interposed and clamped between a pair of electrode base plates whose surfaces have been orientated in mutually intersecting directions thereby to impart optical rotation (a helical structure being imparted to the liquid crystal molecules), and this optical rotation is caused to vary by the application of a suitable electric field and thus utilized for displaying.
The latter apparatus of FE type has such features as lower voltage operation, lower power consumption, and longer serviceable life compared with the former DS type and, for this reason, has wide utility in applications such as various displays and optical shades. In a DS type display device, uniformity of the initial alignment of the liquid crystal is not absolutely necessary from the point of view of operational principle, but in an FE type display device, according to its operational principle, the initial alignment of the liquid crystal is controlled or realigned by the electric field, and the resulting variation of an optical property is utilized, and for this reason, the uniformity of the initial alignment of the liquid crystal (i.e., the uniformity of the alignment when no electric field is applied) is especially important.
Heretofore, various attempts have been made to improve the uniformity of this initial alignment of a liquid crystal. These attempts, however, cannot be considered to have been completely successful in all cases. According to the results of our research, this may be attributed to the reasons set forth below.
First, as a measure known heretofore for obtaining uniformity of the initial alignment of a liquid crystal, there is a method wherein the electrode base plates are rubbed in one direction by a material such as cloth, but by this method, alignments of liquid crystal molecules differ locally, and the uniformity of alignment is not sufficient. Furthermore, the alignment is lost within a short time.
According to another method intended to solve the above described problems, a surface-active agent (surfactant) of a certain kind is additionally used, and the electrode base plates are rubbed in one direction (as disclosed, for example in IBM Technical Disclosure Bulletin Vol. 13, No. 11, Apr. 1971). While the uniformity of alignment is improved to a certain extent by this method, the surfactant does not possess heat resistance and, furthermore, gives rise to deterioration of the liquid crystal material. Then, when an electric field is applied, the surfactant decomposes because of the electric field and undergoes deterioration, and the orientation is destroyed. A further difficulty is that if, in the fabrication of an electro-optical cell after orientation of the electrode base plates, a substance of high softening temperature such as, for example, polyvinylidene fluoride, is used as the sealing material for bonding the pair of electrode base plates, orientation will be lost by the heating temperature at the time of sealing.
Furthermore, a method wherein, for example, the electrode base plates are inclined relative to an evaporation deposition material source, and a material such as silicon oxide or magnesium fluoride is deposited by evaporation thereby to accomplish orientation (referred to as "inclined evaporation orientation method" for convenience) is known (as disclosed, for example, in U.S. Pat. No. 3,834,792) but is accompanied by the problem of weak alignment energy.
With the object of obtaining an electro-optical cell in which the above described various difficulties accompanying the known electro-optical cells for FE type liquid crystal displaying are overcome, we have carried out studies, as a result of which we have made certain findings as set forth below. This invention is based on these findings.